Heretofore, all kinds of electronic devices, which are applied to computers, communication, information appliances, various kinds of display devices or the like, have used a circuit pattern, which comprises a thin film formed on a substrate and made of metal or an insulating material. In order to copy with the advanced information society rapidly growing, such a circuit pattern is required to be highly integrated (be more precise) and to have a larger area.
In general, a method using a photolithography etching process has been adopted in order to form such a circuit pattern. A typical process of this method is shown in FIG. 15 and FIG. 16. As shown in FIG. 15 and FIG. 16, a thin film for forming a circuit pattern is entirely or partly deposited on a substrate, followed by applying a resist and drying the applied resist to form a mask layer in this method. By exposing and developing the mask layer through a mask, a pattern reverse to a circuit pattern (reverse circuit pattern) is formed. After that, a desired pattern is formed through etching and removing of the mask layer in this method. This method is excellent in having mass productivity since the forming precision of a pattern is excellent and since it is possible not only to reproduce the same pattern many times but also to form a plurality of electronic circuits on a single substrate.
However, in this method using a photolithography etching process, many steps are repeated to complete the circuit pattern of an electronic circuit as shown in FIG. 15 and FIG. 16. In the method shown in FIG. 15 and FIG. 16, after a metal thin film 51 is formed on a substrate 50, a mask layer 52 is formed, the mask layer 52 are subjected to exposure, developing treatment, etching and removal, and an insulating layer 53 is formed, followed by forming, exposing, developing, etching and removing a mask layer 54.
In other words, this method needs to have steps as many as about 22 steps comprising film deposition, resist application, drying, exposure, development, etching, removal of a mask layer and the like whenever a circuit pattern comprising a metal thin film and an insulating layer is formed. For this reason, this method has caused a problem in that the production cost is quite high. Further, this method needs to use a large amount of developer, a chemical liquid, such as an etching agent, and a cleaning liquid when such many processes are performed. This has caused problems in that not only the production cost is quite high because of low yield but also the environmental burden, such as liquid waste disposal, which has been recently a matter of serious concern, is quite high.
From this point of view, in order to solve these problems, as the method for shortening the process comprising many steps, there have been performed a method wherein a circuit pattern is directly formed by using, e.g., a metal mask when depositing a material for the circuit pattern, and a method called lift-off wherein before depositing a material for a circuit pattern, a reverse circuit pattern is formed by a mask layer, followed by forming a thin film for the circuit pattern and removing an unnecessary portion of the thin film along with the mask layer.
However, the above-mentioned method using a metal mask has caused a problem in that since a metal mask lifts from a substrate to make a minute gap between the substrate and the metal mask, a material for a circuit pattern soaks into the gap on deposition of a thin film, causing a limitation to improvement in the formation precision of a pattern. Since the adhesion of the metal mask to the substrate is not good, this problem is significant particularly when forming a long continuous pattern. Specifically, the forming precision of the pattern is lowered since the gap increases as the distance from an edge portion of the substrate increases. Accordingly, the lifting of the metal mask contributes to the soaking of the pattern material. Particularly when there is a pattern adjacent a desired continuous pattern, the lifting of the metal mask causes connection of thin films between both patterns. In order to solve this problem, there has been proposed a method wherein a metal mask is brought into contact with a substrate by forming the metal mask from a magnetic material and disposing a magnet on a surface of the substrate remote from the metal mask (see, e.g., Patent Document 1).
The above-mentioned Patent Document 1 shows a method for forming a pattern, which is applied to a method for fabricating an organic electroluminescent display as shown in FIG. 17 through FIG. 19. In Patent Document 1, each of electrode materials is sputtered by using a metal mask 55 and disposing a magnetized member 57 on a surface of a substrate 56 remote from the metal mask 55 as shown in FIG. 17 through FIG. 19. By this arrangement, electrodes, which comprise combinations of a striped organic layer 59 and a striped cathode 60 crossing over anodes 58 in a matrix pattern, are sequentially deposited. This document describes that the magnetized member 57 is formed so as to make the distance between magnetic lines of force as narrow as possible, being capable of coping with the deviation of a pattern and the soaking of a pattern material caused by lift of the metal mask 55, which cause trouble when using the metal mask 55.
With regard to the above-mentioned method called lift-off, there have been many proposals (see, e.g., Patent Documents 2 to 12). Each of Patent Documents 2 to 5 stated above has disclosed a method for forming a mask layer in an inverted tapered shape, which is appropriate for a lift-off method. The above-mentioned Patent Document 6 has disclosed a method for providing thermostability to a resist so as to be capable of withstanding a temperature applied in a subsequent process. Patent Documents 7 and 8 have disclosed other lift-off methods for various kinds of applications.
In general, the above-mentioned lift-off method is a method wherein a mask layer is formed in a reverse circuit pattern on a substrate, a metal thin film is deposited on the entire surface of the substrate, and the mask layer is removed to form a circuit pattern comprising the metal thin film in openings of the mask layer. FIG. 20 and FIG. 21 show a process for forming a mask layer in a reverse circuit pattern by subjecting the mask layer to exposure, development and the like. These figures show a wet lift-off method, which is performed under a so-called wet condition.
In the method shown in FIG. 20 and FIG. 21, a mask layer 62 is formed on a substrate 61, and the mask layer is exposed and developed to form a resist pattern. After that, a metal thin film 63 is formed, followed by removing the unnecessary mask layer 62 and an unnecessary portion of the metal thin film 63. Further, another mask layer 64 is formed on the substrate 61 and is exposed and developed to form a resist pattern, and an insulating layer 65 is formed, followed by removing the unnecessary mask layer 64 and an unnecessary portion of the insulating layer 65.
When a circuit pattern comprising a metal thin film is formed by a lift-off method, it is preferred that portions of the metal thin film formed in openings of the mask layer and portions of the metal thin film formed on the mask layer be not substantially short-circuited. For this reason, the openings of the mask layer are formed in an inverted tapered shape or an overhang shape. Methods for forming a mask layer in an inverted tapered shape or an overhang shape have also been proposed (see, e.g., Patent Documents 9 to 11).
Patent Document 1: JP-A-2002-75639
Patent Document 2: Japanese Patent No. 2989064
Patent Document 3: Japanese Patent No. 3028094
Patent Document 4: JP-A-7-168368
Patent Document 5: JP-A-8-315981
Patent Document 6: JP-A-11-317418
Patent Document 7: JP-A-2002-134004
Patent Document 8: JP-A-11-339574
Patent Document 9: JP-A-56-81954
Patent Document 10: JP-A-1-236658
Patent Document 11: JP-A-7-29846
Patent Document 12: JP-A-2003-287905
Patent Document 13: JP-A-6-13356
Patent Document 14: JP-A-10-20509